Introduction to GIS and mapping in R

About us: Brittany Barker

About us: Carrie Preston

Outline of workshop

• ~20 min: introductory lecture
  
• Parts 1-2: Introduction to sf package
• Part 3: Mapping raster and vector data (ggplot2)
  
• Part 4: Mapping raster and vector date (leaflet)

Source: Zia Ahmed

Elements of Geographic Information System (GIS)

Source: Mierzejowska & Pomyko 2019

Why use R for GIS?

Types of spatial features: vector

• Uses points and/or line segments to identify locations on earth

Different types of vectors (Source: Jakub Nowosad)

R packages for working with vectors

• sf (simple features)
• More convenient and flexible than older sp package
  
• Integration with the tidyverse
  
• Emphasis on the spatial geometry of real world objects

Types of spatial features: raster

• Gridded data where each pixel (cell) is associated with a specific geographical location
  
• Can be numerical or categorical

Vector vs. raster

Types of spatial features: raster

• Multi-layered rasters often called raster stacks
  • Time (day of year, etc.)
  • Different variables

R packages for working with rasters

R packages for working with rasters

• stars(spatiotemporal arrays)

  • Has a few abilities lacking in terra (e.g., non-regular shapes)

  • Many sf functions work with stars objects Not as well documented

Source: Edzer Pebesama

Layering

• Oftentimes we want to make maps with multiple spatial features

In comes ggplot

• Uses layered approach to create graphics (The Grammar of Graphics)
• You provide the data, tell ggplot2 how to map variables to aesthetics, what graphical elements to use, and it takes care of the details

Source: R for the rest of us

In comes ggplot

• ggplot2 can plot all sorts of data, including rasters and vectors

Source: www.icertglobal.com

Example: a ggplot combining raster and vectors

• Map of the phenology and potential distribution for invasive emerald ash borer
  
• What are the vectors vs. rasters in this map?

Interactive web maps using leaflet

• ggplot2 creates static maps, whereas leaflet creates interactive maps
  
• Used for maps in web pages, including Shiny web apps

Source: https://rstudio.github.io/leaflet/articles/shiny.html

Coordinate reference system (CRS)

• Coordinate system: X, Y grid upon which data is overlaid
  
• Units: horizontal & vertical units of grid (e.g., km, miles)
  
• Datum: defines origin used to place the coordinate system in space
  
• Projection: equation used to flatten objects to 2D surface

Source: Bill Huber, https://www.directionsmag.com

Why is the CRS important?

• Framework for precisely measuring locations on earth’s surface
  
• Errors or mistakes occur when using inconsistent CRS
  
• For example, mismatched features and incorrect distance measurements

Source: opennews.org

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